As a device for measuring the position of an object that exists at a distant point, a radar apparatus is known.
A radar apparatus emits a wave, such as an electromagnetic wave or acoustic wave, into space, receives a wave that is reflected from an observation object and returned back, and analyzes the returned wave to measure a distance from the radar apparatus to the object and an angle of the object.
Among radar apparatuses, a weather radar apparatus is known. A weather radar apparatus observes a very small aerosol floating in the atmosphere, and measures, as a wind velocity, a velocity at which an aerosol moves from an amount of phase rotation of a wave that is reflected from the aerosol and returned back.
Particularly, among weather radar apparatuses, a laser radar apparatus using light as an electromagnetic wave is used as a wind direction and wind velocity radar because the laser radar apparatus has a very small divergence of a beam emitted and therefore can observe an object with a high angular resolution (for example, refer to Nonpatent Literature 1).
A typical laser radar apparatus emits laser light into the atmosphere, after that, receives the laser light which is reflected by an aerosol in the atmosphere and then returns thereto, i.e., the laser light which has received a Doppler frequency shift that depends on the moving velocity of the aerosol in the atmosphere, and performs heterodyne detection on the laser light and local light, thereby detecting a Doppler signal corresponding to a wind velocity.
Such a laser radar apparatus is generally called a Doppler lidar, and divides a laser light beam which is reflected by an aerosol at each altitude in the atmosphere and then returns thereto into laser light beams with respect to time, and performs coherent integration in very small spaces within each of range bins corresponding to the laser light beams separate with respect to time.
When performing the coherent integration within each of the range bins, the laser radar apparatus needs to shorten a unit time with which the laser radar apparatus divides the laser light beam in order to grasp spatial variations in wind velocities in detail, because the unit time with which the laser radar apparatus divides the laser light beam corresponds to the distance resolution. However, because the signal amount acquired decreases when the time required for the coherent integration is shortened, there is a trade-off relationship between the time required for the coherent integration and the distance up to which observations can be performed, i.e., the distance becomes short with decrease in the time required for the coherent integration.
In order to prevent the distance up to which observations can be performed from becoming short even if the time required for the coherent integration is shortened, there is a method of improving the signal to noise ratio (referred to as the “SNR” from here on) by performing the coherent integration, after that, performing a Fourier transform on a coherent integrated result within each of the range bins, and then performing incoherent integration on a result of the Fourier transform. It is known that the SNR is improved generally by √N when the incoherent integration is performed N times (for example, refer to Patent Literatures 1 and 2).
When acquiring a spectrum signal by performing coherent integration within each of the range bins, a typical laser radar apparatus specifies a Doppler shift amount which is a frequency at which the spectrum signal has a maximum, and calculates a wind velocity from the Doppler shift amount.
Therefore, if a Doppler shift amount which is a frequency at which the spectrum signal has a maximum can be specified with a high degree of accuracy, the calculation accuracy of a wind velocity can be improved, but an area (high SNR area) of distances with a high SNR and an area (low SNR area) of distances with a low SNR usually coexist.
Although in the high SNR area it is possible to calculate a wind velocity correctly because the spectrum signal has a higher peak than noise, in the low SNR area noise may have a higher peak than the spectrum signal. Therefore, in the low SNR area, the peak of noise may be detected erroneously as the peak of the spectrum signal, and an incorrect wind velocity may be calculated.
In the following Patent Literature 3, a laser radar apparatus that, when searching for the peak of a spectrum signal, determines a standard deviation of wind velocities between the gate (distance) of a high SNR area and that of a low SNR area adjacent to the high SNR area, and determines a peak search scope within which to search for the peak of the spectrum signal by using the standard deviation is disclosed.
Because, as a result, the range within which to search for the peak of the spectrum signal is limited, the probability that the peak of noise is detected erroneously as the peak of the spectrum signal is decreased.